People who notice a friend packing on pounds might want to steer clear if they value a sleek physique.

A new study finds that when the scale reads "obese" for one individual, the odds that their friends will become obese increase by more than 50 percent.

The study, published in the July 26 issue of the New England Journal of Medicine, suggests that obesity is "socially contagious," as it can spread among individuals in close social circles. The likely explanation: A person's idea of what is an appropriate body size is affected by the size of his or her friends.

Conversely, the researchers found that thinness is also contagious.

"Social effects, I think, are much stronger than people before realized," said co-author James Fowler, a social-networks expert at the University of California-San Diego. "There's been an intensive effort to find genes that are responsible for obesity and physical processes that are responsible for obesity, and what our paper suggests is that you really should spend time looking at the social side of life as well."

An outside expert on social networks called the new research impressive, particularly in showing a causal link between obesity and friends. However, he cautioned that the evidence for the effect extending out to friends' friends, and so on, is weaker.

"The suggestion in their paper is that obesity sort of spreads through the network as if it were some kind of epidemic, some kind of contagious disease," said Duncan Watts, who studies social networks at Columbia University. While this is plausible, he noted, the current research doesn't provide direct evidence for this phenomena.

Social networks

Research has shown that peers influence each other's health behaviors. One past study showed that teens associating with friends who smoke and drink were more likely to take up the behaviors. However, no past research has looked at how the impact extends to friends' friends and beyond.

In the new study, Fowler and Nicholas Christakis of Harvard Medical School analyzed health data collected between 1971 and 2003 from more than 12,000 adults who participated in the Framingham Heart Study, an ongoing cardiovascular study. Participants provided contact information for close friends, many of whom were also study participants, resulting in a total of 38,611 social and family ties.

The researchers found that if a participant's friend became obese over the course of the study, the chances that the participant also became obese increased by 57 percent. Among mutual friends (both individuals indicate the other is a "friend"), the chances nearly tripled.

Among siblings, if one becomes obese the likelihood of their sister or brother becoming obese increases by 40 percent. Among spouses there is a 37 percent increased risk.

Gender also affected the degree of "obesity contagion." In same-sex friendships, individuals had a 71 percent increased risk of obesity if a friend became obese. If a guy's brother is obese, he's 44 percent more likely to also become obese. Among sisters, the risk was 67 percent.

Fat factors

Other studies have suggested that obesity might be physically contagious, possibly passing from one person to another by virus. But that idea has not been firmly supported. The new study doesn't address this possibility but instead looked at mindsets and attitudes as the controlling factors.

Fat-fueling factors were taken into consideration. For instance, the researchers made sure the effect wasn't a case of "birds of a feather flocking together." Body measurements were taken throughout the study period, showing when individuals became obese and whether they began the study with obese readings.

"It's not that obese or non-obese people simply find other similar people to hang out with," Christakis said. "Rather, there is a direct, causal relationship."

They also ruled out the idea that an outside factor, and not the friendship, caused the fatness. If an environmental factor were affecting both individuals in a friendship, then it shouldn't matter whether individuals are mutual friends or just one individual labels the other as a friend.

The study, however, found that it does matter which way the friend arrow points: If subjects named an obese person as a friend, they tended to be affected by that person's obesity.

But when the person on the receiving end did not label the first person as a friend, there was no "obesity contagion" effect in the other direction. The distinct variable here is who calls whom a "friend."

"The fact that it only has an effect when I think you're my friend is very strongly suggestive to me," Watts said. "That's about as good as you can do in terms of identifying a causal relationship."

Perhaps friends just spend a lot of time together and so would eat similar foods and engage in the same physical activities. But they found the results held no matter the geographic proximity of friends.

"So friends that are thousands of miles away have just as large an impact on you as friends who are right next door," Fowler told LiveScience.

The scientists suggest the findings can be explained if friends are influencing one another's norms for body weight.

"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size," Christakis said. "People come to think that it is OK to be bigger since those around them are bigger, and this sensibility spreads."

Bulging waistlines

In the past 25 years, obesity among U.S. adults has shot from 15 to 32 percent. The new study reveals friends could be feeding the fat epidemic, along with our large-serving, high-calorie, fast-food lifestyles.

"We show that one person's behavior ripples through the network to have an impact beyond those first-order friendships," Fowler said. "So we're talking about dozens of people that are affected by one person's health outcomes and health behaviors."

He added, "And that needs to be taken into account by policy analysts and also by politicians who are trying to decide what the best measures are for making society healthier."